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Full text of "Handbook Of Chemical Engineering - I"

GRADING AND SCREENING
223
For human and animal consumption salt is graded to obtain varying rates of solubility. For household use a quickly dissolving product is desired, while for layering meat the salt is desired in a condition such that it will dissolve slowly. This requires that the salt for household purposes be fine but not so fine that it loses its crystalline character for it would cake too readily in the receptacles for holding it in a damp climate. If fine salt were used for preserving meat it would quickly dissolve in the moisture of the flesh and run to waste permitting the entry of decaying agencies. A very coarse salt is consequently used for layering meats, intermediate sizes between this and the finest being used where greater rates of solubility are desired.
Miscellaneous Modes of Grading.—A means for separating shrunken and discolored beans from perfect white beans is disclosed in a recent patent (U. S. Patent 1,080,088, Dec. 9, 1913). By this patent means are provided for bringing
JMercury Bath
FIG.  1.—Grading by weighing by means of a hinged spout.
the beans singly to a position in front of a hinged spout. Means are provided for reflecting all the light from the beans onto a selenium cell which consists of narrow strips of selenium joining broad brass plates the whole forming a portion of an electric circuit. If the hinged spout be supported by the attraction of a magnet which is part of the selenium cell circuit, then when the perfect beans pass through the apparatus, since the resistance of the circuit will be diminished from the greater light impinging on the cell, the spout will be raised to the highest point and if a receptacle is placed in the proper position below the edge of the spout the perfect beans will fall into it. If other receptacles or compartments are placed behind the first a gradation can be obtained from perfect white beans to dark, shrivelled beans. Greenish-yellow rays lower the resistance of selenium the most. The device described may offer a means of grading according to color and may have some application in concentrating ores.
Grading by One Dimension Only.—A silk-sorting machine is one for grading threads according to thickness and winding them upon the proper bobbins. The proper bobbin is presented to the thread by the action of a lever which is governed by the thickness of the thread passing between gage rollers.
Grading by Weighing.—Grading by weighing is only adapted to relatively large individual pieces. Each part of such a device must have means for feeding the individual piece singly and the interval of time between pieces must be suffr